1. Field of the Invention
The present invention relates to a method for inspecting lithography multilayer masks for small defects, and to a defect inspection apparatus using the method.
2. Description of the Prior Art
Extreme ultraviolet (EUV) lithography (EUVL), in which an EUV light of 13 nm wavelength is employed as an illumination light, is the most promising lithography candidate for fabricating integrated circuits with a feature size of 70 nm or below. As such, related technologies are being developed.
One such technology that needs to be developed is that of inspection of defects on a mask. Since all materials absorb EUV light strongly, EUVL utilizes a reflection mask. To obtain high reflectance for EUV light, a multilayer structure consisting of several tens of pairs of layers of Si and Mo, each approximately 3-nm thick, are formed by vapor deposition on an optical device surface. An EUVL mask is a reflective mask on top of the multilayer reflector on which the device circuit pattern is defined by depositing absorbing material, and the size of the mask is about 140 mm by 140 mm. It is considered necessary to detect defects having a size of the order of 30 nm.
In existing lithography technologies, in which ultraviolet lasers are used as the light source, transmission type masks are employed and they are inspected using a visible laser-beam illumination. There have also been attempts to use laser-beam illumination in the case of EUVL reflective masks. However, the fact that the patterns to be inspected are becoming smaller, and that there is not much difference between the reflectance of the substrate Mo/Si multilayer and that of the absorbing material that is to be inspected for pattern defects, makes it difficult to inspect reflective masks for defects by a visible layer. This is explained in further detail, as follows.
When we designate Rdef as the reflectance (transmittance) of the defect that is tile target of the inspection, and the Sdef as the defect area and Rpix as the background reflectance (transmittance), and Spix as the area thereof (pixel size), when a mask is illuminated by a photon density n, the amplitude of a signal Idef when there is a defect, and of a signal Ipix when there is no defect, will be as follows:Idef=nRdefSdef+nRpix(Spix−Sdef), Ipix=nRpixSpixTo be able to discriminate the two signals with sufficient accuracy, the difference between the signal amplitudes is to be at least three times larger than the shot-noise standard deviation σ, thus:|Idef−Ipix|≧3(Idef)1/2+3(Ipix)1/2Therefore,|(Idef)1/2−(Ipix)1/2|≧3As such, the requisite number of photons is as follows.(nSdef)1/2≧3Rpix1/2((Rdef/Rpix)+(Spix/Sdef−1))1/2+(Spix/Sdef)1/2/(Rdef−Rpix)  (1)
With respect to visible light and ultraviolet light, a Mo/Si multilayer reflector acts as a metal reflector, and the mask pattern is also metal, so there is not much difference in reflectance between the two. The difference in reflectance is particularly small in the case of far ultraviolet light of wavelength 200 nm or below. For example, assuming a defect reflectance of Rdef=0.54 and a background substrate mask reflectance of Rpix=0.39, if the target defect size is 30 nm and the pixel size is 200 nm, from equation (1), we get the following.(nSdef)≧27,800  (2)
When the number of photons needed for illumination is 28,000, because the area of a 140 mm by 140 mm size mask is 2 E13 times of the defect area Sdef, the number of photons needed to illuminate the whole mask will be 6E17. If the energy of one photon is 6 eV, the total energy would be 0.6 J. In order to complete inspection of the whole mask in three hours, if a total exposure time of one hour is allowed, an average illumination power of 0.2 mW would be required.
Under the above conditions, Idef=486,000 and Ipix=481,000, then, the difference between which is merely 1%. If the substrate is perfectly uniform, such as in the case of a Si wafer, inspection at the above power is possible, but in the case of a lithography mask on which complex patterns are formed, the intensity of reflection from the mask could well vary in the order of a percentage depending on location. This means that determining the presence or absence of defects necessitates complex processing by taking into consideration the fact that the reflection intensity varies on the mask from place to place, in addition to which the number of photons needed to facilitate detection of signals from defects is several orders greater than the number calculated by equation (2). Decreasing the is amount of tune allocated to exposure to reduce the time required for processing the data, results in an increase in the power that is needed. How much power is actually required is something that has to be determined through future empirical investigation, but it is feared that more than a watt of power will be required.
As can be seen from equation (1), the number of photons required can be decreased by bringing the size of the pixel Spix closer to the size of the target defect Sdef. Assuming the same reflectance, if a pixel size of 90 nm was used, then (nSdef)≧5,700, meaning it would be possible to reduce the required power to one-fifth.
However, because light cannot be focused to below its diffraction limit, the wavelength of the illuminating laser beam has to be shortened in order to decrease the size of the beam, meaning the size of the pixel. However, it is not easy to decrease the laser wavelength to below 200 nm. Because it is so difficult to achieve continuous laser oscillation at or below a wavelength of 250 nm directly, attempts have been made at achieving shorter wavelengths by the wavelength conversion of 266-nm light, but the conversion decreases the power, in addition to which, based on such factors as the power resistance of the wavelength conversion crystals, at present it is only possible to obtain power in the microwatt to sub-milliwatt order. Even when the power needed for inspection is reduced by reducing a pixel size using a short wavelength laser, if decrease of the available power for shorting wavelength of the laser wavelength is greater than the decrease of the power for inspection, it is better not to reduce the laser wavelength. That is to say, detecting ultrasmall defects is difficult when the illumination used is of a wavelength that provides poor contrast between the reflectance of the substrate and that of the pattern.
If the wavelength of the inspection light is the peak wavelength of the reflection spectrum of the multilayer mask, the contrast ratio between the signal from the inspection target and the signal from the background can be large to make detection of small defects easier. However, if the inspection is performed in the bright field configuration so that Rdef is small and the substrate reflectance Rpix is large, as can be understood from equation (1), no major improvement in contrast ratio can be achieved. In order to achieve a jumping improvement in the contrast ratio, it is important to increase the signal from defects and decrease the signal from the background. If, for example, a defect produces an effective reflectance Rdef of 0.6 and the background effective reflectance Rpix can be reduced to 0.001, even if the size of the pixel is as large as 3 μm, equation (1) gives the number of photons needed to detect a 30-nm defect.(nSdef)≧1,030  (3)in this case, Idef=10,920 and Ipix=10,300, so the difference in intensity is as large as 6%, facilitating the determination of whether there is or is not a defect.
In the case of the use of the laser described above, at (nSdef)≧27,800, some 30 times more photons are required, and, moreover, there is only a 1% difference between the intensities of signals from a pixel that includes a defect and a pixel that does not include a defect. It also has to be noted that that was the calculation in the case of a 200-nm pixel size, which is just 1/200 the size of a 3 μm pixel, This shows the great effectiveness of decreasing the background signal.
Large value for the reflectivity Rdef by defects and small reflectivity Rpix for the background can be achieved in tie dark-field configuration, in which specularly reflected light is blocked.
When U1 is the light wave observed at the observation point when a masking shield is provided between the light source and the observer, and U2 is the light wave when the aperture is placed in precise registration with the masking shield, and U0 is the light wave when there is nothing therebetween, based on the Babinet principle, U1+U2=U0. If a disposition is used that blocks specularly reflected light, then U0=0, therefore U1=−U2, in which case the intensity |U1|2 of the scattered light signals shielded by a defect is equal to the intensity |U2|2 of the light transmitted from the aperture in precise registration with the defect. That is, in the dark-field observation, Rpix will be substantially zero, and Rdef, which determines scattered light intensity from a defect, will be equal to the reflectance of the substrate mask.
Yi et al. conducted tests using the dark-field configuration (J. Vac. Sci. Tech. B18 (2000) 2930). A microbeam measuring 2.5 μm by 4 μm was formed by focusing a synchrotron beam using a grazing incidence KB mirror configuration on which reflected 13-nm EUV fell incident at a glancing angle. The beam was used to illuminate programmed defects, and an MCP detector was used to detect scattered light from the defects. The MCP detector, which had an outside diameter of 44 mm and a central opening with a diameter of 4.7 mm, detected scattered light, without detecting specularly reflected light. The collection solid angle of the detector was 0.068 rad. With this arrangement, they claimed 60-nm defects on the multiplayer were detected. Thus, with the dark-field observation, it was possible to obtain a great improvement in the detection sensitivity of small defects, enabling actual detection of a 60-nm defect in a large pixel size of 2.5 μm by 4 μm. That is, it was possible to measure a small defect with an area ratio of 2800:1.
However, they reported that it took 30 hours to scan a region of 1 cm2. To be of practical utility, it is necessary to be able to inspect a 140 mm by 140 mm mask in two or three hours. This means we need to improve the inspection speed by three orders of magnitude. Moreover, even if a high speed were to be realized, their synchrotron light facility would be too large and costly for practical lithographic application.
In view of the above drawbacks of the prior art, an object of the present invention is to provide a method and apparatus for inspecting multilayer masks for small defects difficult to detect with means using visible or ultraviolet lasers, with inspection speed faster more than three orders of magnitude of the speed reported by the prior works using a synchrotron source.
Another object is to provide a method and apparatus for inspecting multilayer masks for defects that employ a light source that is compact and can be readily utilized by anyone.